The following six types of method for continuously casting steel are conventionally known:
1. The single-curvature/spot-straightening type, using a curved mold, which comprises casting steel in said curved mold, guiding an unsolidified cast strand with a solidified shell formed on it while bending said cast strand along a curvature having the same radius as that of said curved mold, then accelerating and completing the solidification of said unsolidified cast strand while straightening said cast strand thus bent in a substantially horizontal direction at a single point, and cutting and transporting said cast strand completely solidified in the horizontal position;
2. The infinite-point bending/gradual-straightening type, using a curved mold, which comprised casting steel in said curved mold, guiding to some extent an unsolidified cast strand with a solidified shell formed on it while bending said cast strand along a curvature having the same radius as that of said curved mold, then accelerating and completing the solidification of said unsolidified cast strand while gradually straightening said cast strand thus bent in a substantially horizontal direction by increasing the radius of curvature of said unsolidified cast strand thus bent at multiple points or at infinite points, and cutting and transporting said cast strand completely solidified in the horizontal position;
3. The fully vertical type, using a straight mold, which comprises casting steel in said straight mold, accelerating and completing the solidification of an unsolidified cast strand with a solidified shell formed on it in the vertical position while guiding said unsolidified cast strand substantially vertically downward, cutting said completely solidified cast strand in the vertical position, and then transporting it;
4. The vertical-solidification/single-curvature/spotstraightening, using a straight mold, which comprises casting steel in said straight mold, accelerating and completing the solidification of an unsolidified cast strand with a solidified shell formed on it in the vertical position while guiding said unsolidified cast strand substantially vertically downward, bending said completely solidified cast strand at a constant radius of curvature from the vertical position, then straightening said cast strand in a substantially horizontal direction at a single point, and then cutting and transporting said cast strand in the horizontal position;
5. The vertical-unsolidification/single-curvature/spotstraightening type, using a straight mold, which comprises casting steel in said straight mold, guiding to some extent an unsolidified cast strand with a solidified shell formed on it substantially vertically downward, then bending said unsolidified cast strand at a constant radius of curvature from the vertical position, accelerating and completing the solidification of said unsolidified cast strand while straightening said cast strand in a substantially horizontal direction at a single point, and cutting and transporting said completely solidified cast strand in the horizontal position;
6. The vertical-unsolidification/infinite-point bending/gradual-straightening type, using a straight mold, which comprises casting steel in said straight mold, guiding an unsolidified cast strand with a solidified shell formed on it substantially vertically downward within a range from 2 to 3m from the meniscus of molten steel in said straight mold, then gradually bending said unsolidified cast strand in the vertical position by successively decreasing the radius of curvature at infinite points to a constant curvature, then accelerating and completing the solidification of said unsolidified cast strand while slowly straightening it in a substantially horizontal direction by increasing said constant radius of curvature at infinite points and cutting and transporting said completely solidified cast strand in the horizontal position.
The conventional types of continuous casting of steel mentioned above involve the following problems: in types (1), (2), (5) and (6), inclusions in molten steel teemed into a mold penetrate, together with molten steel, deeply into the curved portion of an unsolidified cast strand because the unsolidified cast strand is guided while being bent. On floating up by buoyancy, inclusions are trapped on the inner surface of the curved portion of the solidified shell of the unsolidified cast strand and cannot in many cases float up completely to the meniscus of molten steel in a mold. This causes the formation of a localized zone of inclusions at a position in the interior of the completely solidified cast strand.
In types (3) and (4) mentioned above, on the other hand, a localized zone of inclusions as described above is never formed at a position in the interior of a completely solidified cast strand because the cast strand is held in the vertical position up to the completion of solidification. However, the increased height from the meniscus of molten steel in the mold up to the solidification completing point of the unsolidified cast strand, i.e., up to the crater top in the unsolidified cast strand causes increase in the static pressure of molten steel near the solidification completing point in the interior of the unsolidified cast strand. The diameter of rollers guiding the unsolidified cast strand should therefore be enlarged sufficiently to cope with this increased static pressure of molten steel. This increase in the guide roller diameter leads to a longer pitch between rollers, which in turn results in a larger amount of bulging of the unsolidified cast strand between rollers. This causes formation of a dense center segregation in the cast strand. Reduction of the cast strand at a relatively large draft near the solidification completing point in the cast strand, if applied with reduction rolls with a view to straightening this large bulging and eliminating the center segregation, produces many inner cracks on the liquid-solid interface of molten steel in the cast strand.
In types (1) and (5) mentioned above, furthermore, a cast strand is bent at a constant radius of curvature and then straightened in a substantially horizontal direction at a single point in its unsolidified state. Bending and straintening thus applied cause a too large amount of deformation strain of the cast strand, thus resulting in the formation of many surface and inner cracks.
C, Mn, S and other constituents are segregated in general in the unsolidified molten steel near the solidification completing point of a continuously cast strand. In any of types (1) to (6) mentioned above, the formation of a dense center segregation zone at the center of the cast strand cross-section is inevitable because of the bulging between rolls near the solidification completing point of the cast strand.